[core] support for 'x-enum-varnames' for enums in arrays

[jmini]

Description

While I was working on #1693 I have discovered an other case that is not working with x-enum-varnames: when the enum is defined in a list.

openapi-generator version

4.0.0-SNAPSHOT

OpenAPI declaration file content or url

    SomeObject:
      type: object
      properties:
        someProperty:
          type: array
          items:
            type: integer
            format: int64
            enum:
              - 20
              - 30
              - 40
            x-enum-varnames:
              - TWENTY
              - THIRTY
              - FORTY

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[jmini]

PR #1703 is merged